1. Field of the Invention
The present invention relates to communication systems, optical line terminals (OLTs), and congestion control methods, and more particularly, to a communication system, an optical line terminal, and a congestion control method for preventing Internet group management protocol (IGMP) requests generated when video service viewing users switch the channels, from congesting in the optical line terminal in a passive optical network (PON).
2. Description of the Related Art
To transmit and receive large-size data and image signals through communication networks, the networks have been improved to have a higher rate and a wider band. PONs, which are defined by “Gigabit-capable passive optical network (GPON)”, Recommendation G. 984, Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), and other rules, have been introduced.
FIG. 1 shows the structure of a network in a PON system.
The network in the PON system includes, for example, a video service provider 17, an Internet service provider 18, a voice service provider 19, a router 16-1, an OLT 10, a core optical fiber 11, an optical splitter 12, a plurality of local optical fibers 13, optical network terminals (ONTs) 14-1 to 14-3, and terminals 15-1 to 15-3.
As shown in FIG. 1, for example, in the PON system, an upstream (service-provider-side) communication network is connected to the OLT 10 through the router 16-1; and the ONTs 14-1 to 14-3, which accommodate a plurality of subscriber terminals, are connected to the OLT 10 through a PON which includes the core optical fiber 11, the optical splitter 12, and the plurality of local optical fibers 13. The upstream network includes, for example, the video service provider 17, the Internet service provider 18, and the voice service provider 19. The ONTs 14-1 to 14-3 are respectively connected to the terminals 15-1 to 15-3 (such as set top boxes (STBs), telephones, and PCs).
Client data (such as video data sent by the video service provider 17, data sent by the Internet service provider 18, and voice calls and other service data for general subscribers sent by the voice service provider 19) is mainly transferred between the OLT 10 and the ONTs 14-1 to 14-3 (this part is also called a PON zone).
As broader bands have been used, IP multicast technologies, which distribute an identical content, such as video data, to a specified large number of users efficiently, have attracted attention. As such a technology, IGMP, recommended by the Internet Engineering Task Force (IETF), has been widely spread. In IGMP, a router that supports multicast (hereinafter also called a multicast router), such as the router 16-1, manages subscriber terminals, such as the ONTs 14-1 to 14-3, serving as members of each multicast group in a connection network and distributes identical data to a specified large number of subscriber terminals in each multicast group.
FIG. 2 is a sequence diagram for IGMP requests and video data stream in video multicast.
In multicast distribution using IGMP, an STB serving as a subscriber terminal uses IGMP to send a join request (IGMP join) 21 or a leave request (IGMP leave) 26 (these requests may be called just IGMP requests) for joining or leaving from a multicast group to a multicast router or to a video server; multicast group management apparatuses, such as an ONT, an OLT, and the multicast router, manage joins 22-1 to 22-3 to the multicast group and leaves 27-1 to 27-3 from the multicast group; and various types of multicast data is sent from a data distribution apparatus, such as a video server, only to the members (user terminals) participating in the multicast group with the members of the multicast group held at the multicast group management apparatuses being referenced.